Capacitive touch sensors are used as a user interface to electronic equipment, for example, computers, mobile phones, personal portable media players, calculators, telephones, cash registers, gasoline pumps, etc. In some applications, opaque touch sensors provide soft key functionality. In other applications, transparent touch sensors overlay a display to allow the user to interact, via touch, with objects on the display. Such objects may be in the form of soft keys, menus, and other objects on the display. The capacitive touch sensors are activated (controls a signal indicating activation) by a change in capacitance of the capacitive touch sensor when an object, for example, a user's finger tip, causes the capacitance thereof to change.
Today's capacitive touch sensors typically come in one of two varieties: single-touch and multi-touch. A single-touch sensor detects and reports the position of one object in contact with the touch sensor. A multi-touch sensor detects the position of one or more objects in simultaneous contact with the touch sensor, and reports distinct position information related to each object. While both single- and multi-touch capacitive sensors have been around for some time, products using single-touch capacitive sensors have, until recent years, been much more prevalent. As a result, many off-the-shelf touch screen controller products, including integrated circuits and the like, are available for use in single-touch sensor systems.
For example, in a touch sensor utilizing an X-Y or grid-like arrangement of electrodes on different layers, current off-the-shelf touch controllers use various forms of self capacitance measurements to determine the location of touch. A self capacitance measurement measures the capacitance of individual electrodes within a touch sensor and determines the position of touch based on the electrode(s) experiencing the most significant change in capacitance. For example, using an X-Y grid, a touch controller iterates through each of the X-axis and Y-axis electrodes, selecting one electrode at a time and measuring its capacitance. The position of touch is determined by the intersection of (1) the X-axis electrode experiencing the most significant capacitance change and (2) the Y-axis electrode experiencing the most significant capacitance change.
Currently, self capacitance measurements may be taken by, for example, a relaxation oscillator-based measurement or a charge time-to-voltage measurement. For example, the Capacitive Sensing Module (CSM) on certain PIC microcontrollers manufactured by Microchip Technology, Inc. implements a relaxation oscillator circuit for measuring self capacitance in a single-touch sensor system. In addition, the Charge Time Measurement Unit (CTMU) on certain PIC microcontrollers manufactured by Microchip Technology, Inc. implements a charge time-to-voltage circuit for measuring self capacitance in a single-touch sensor system. Both the CSM and CTMU have gained widespread acceptance for use in single-touch sensor systems, and both provide a reasonably fast system response time.
However, traditional self capacitance methods (for example, those used by the CSM and CTMU) cannot support the tracking of multiple simultaneous (X,Y) coordinates, as required in a multi-touch sensor system. For example, in a 16×16 electrode grid, the simultaneous touch by one object at position (1,5) and a second object at position (4,10) leads to four possible touch locations: (1,5), (1,10), (4,5), and (4,10). A self-capacitance system is able to determine that X-axis electrodes 1 and 4 have been touched and that Y-axis electrodes 5 and 10 have been touched, but it is not capable of disambiguating to determine which two of the four possible locations represent the actual touch positions.
Multi-touch capacitive sensors, on the other hand, have only recently gained popularity as a result of technological advancements (e.g., faster processors, lower power consumption requirements, etc.) that have enabled the mainstream deployment of sophisticated personal media devices, cell phones, and the like. While new multi-touch touch sensor controller products are becoming available, they tend to rely on new methods developed specifically in response to the increased demand for multi-touch capability. However, these methods are not as mature as those employed for single-touch sensor systems, resulting in less familiarity and longer development times for those wishing to produce a multi-touch sensor.
Therefore, it is desirable to have a method for detecting multiple touches in a multi-touch sensor system that is easy to implement and that requires minimal time to develop. According to the teachings of this disclosure, this is accomplished by improving methods previously used in single-touch sensor systems so that they may be used in multi-touch sensor systems without the aforementioned ambiguity problem. More specifically, the relaxation oscillator-based and charge time-to-voltage-based measurements described above may be improved and adapted to work in a multi-touch sensor system utilizing mutual capacitance measurements.